1. Field of the Invention
The present invention relates generally to a circuit for controlling voltage spikes and ringing during turn-off of half-bridge rectifiers in bridge-type pulse width modulated direct current - direct current (PWM DC-DC) converters. More particularly, it relates to such a circuit for controlling voltage spikes and ringing which redirects energy in a leakage inductance of a transformer to a load in a lossless manner using active switches. Most especially, it relates to such a circuit for controlling voltage spikes and ringing which permits use of lower voltage rectifiers and significantly reduces conduction losses.
2. Description of the Prior Art
When output rectifiers of a PWM DC-DC converter with transformer isolation turn off, voltage spikes and high-frequency ringing are induced across the rectifier terminals due to the presence of leakage inductances in the transformer. The voltage spikes impose severe stresses on the rectifiers and force one to use higher voltage rectifiers, resulting in higher conduction losses. Traditionally, resistance--capacitance (R-C) or resistance--diode--capacitance (R-D-C) snubber circuits are used across the rectifiers to limit the voltage spikes. The capacitance chosen in an R-C or R-D-C snubber is usually much larger than the junction capacitance of the rectifier, so that the majority of the leakage inductor current is diverted through the snubber, resulting in a smaller voltage overshoot. The resistor provides necessary damping to reduce ringing, but it dissipates energy. The energy stored in the capacitors has to be redirected to the load each cycle when the rectifier turns on. This introduces additional losses in the resistor unless a diode is used across it (R-D--C). Tradeoffs are usually necessary in the choice of R and C values, since a large capacitance reduces voltage spikes while increasing the power dissipation, and a smaller resistance reduces losses while decreasing the damping effects. An optimum design is hard to achieve.
Various clamping and snubbing circuits have been proposed to limit or eliminate voltage spikes across rectifiers. It is known to provide a R-D-C clamp circuit for a full-bridge rectifier as shown in FIGS. 1 and 1A in the present application. Although such a circuit can effectively limit the voltage across the rectifier, significant energies are lost in the discharging resistance. A known lossless active snubber circuit totally eliminates voltage overshoot and ringing across a full-bridge rectifier. While this circuit has many virtues and requires a few extra components, it is not clear whether and how it can be implemented in a half-bridge rectifier, which most low to medium voltage power converters have adopted.